The present invention relates to an axial pole motor. Such a motor typically comprises a stator having a number of poles with inductive windings wound on the poles and a rotor which faces the stator across a plane perpendicular to the rotor axis. Rotation of the rotor is controlled by energizing the stator windings to provide a rotating magnetic field.
According to a first aspect of the present invention, an axial pole motor comprises a stator and a rotor which face one another across a plane perpendicular to the rotor axis, the stator having coil windings substantially uniformly circumferentially spaced and lying in planes substantially perpendicular to the rotor axis defining a number of stator poles, the rotor also including a number of rotor poles comprising one or more permanent magnets, wherein the number of stator poles is different from the number of rotor poles and the stator poles and rotor poles are of substantially the same size and shape so as to provide a pole overlap pattern in which when any one rotor pole completely covers a facing stator pole, the difference between the number of rotor poles and the number of stator poles which exist between that rotor pole and the next rotor pole which completely covers a facing stator pole is one.
In the present invention the rotor poles in an axial pole motor comprise one or more permanent magnets. The rotor may be formed from a single toroidal magnet or, as is preferred, from a number of separate magnets. The permanent magnets increase the torque and power output of the motor. Importantly, the size and shape of the stator poles and rotor poles are substantially the same and the number of stator poles is different from the number of rotor poles. This ensures that maximum use is made of the available surface area of the stator and rotor and provides a pole overlap pattern in which when any one rotor pole completely covers a facing stator pole, the difference between the number of rotor poles and the number of stator poles which exist between that rotor pole and the next rotor pole which completely covers a facing stator pole is one. The arrangement also provides a high detent torque that is a strong cogging action. The motor is able to generate high torques and/or high power within frame sizes which are much smaller in comparison to conventional axial or radial motors.
The motor is especially suitable as a direct drive for a ram or screw, replacing conventional hydraulic drives. A separate gearbox is not required. The design allows for synchronous operation from a mains supply.
Preferably, the axial pole motor comprises a pair of stators with the rotor being positioned between the two stators. This double sided design ensures that the motor is exceptionally stable in that magnetic forces which develop in the stators are balanced.
Preferably, the permanent magnets are secured within a rotor support frame of a non-magnetic material which ensures the separation of adjacent magnets. The rotor support frame may be cast, molded or machined, or a combination of all three, in one or more parts.
In a preferred example, the rotor support frame comprises a non-magnetic hub having circumferentially spaced arms and an outer rim and the permanent magnets are secured in position within the rotor support frame using a potting compound. Alternatively, the rotor support frame may be formed from a plastic material which is injection molded with the permanent magnets in situ.
Each of the opposite faces of the rotor may be provided with a skin of non-magnetic material so that the permanent magnets are completely encased. A suitable material for such a skin is fibreglass. However, to minimize the air gaps in the assembled motor which exist between each of the opposite faces of the rotor and a respective stator, the faces of the permanent magnets may remain exposed. In this case, the faces may be plated to prevent corrosion.
The permanent magnets which form the rotor poles may be cast, molded or cut and may be pre-magnetized or magnetized in situ. Examples of suitable magnetic materials include samarium cobalt and neodymium iron boron. Magnetizing the rotor poles in situ has the advantage that unmagnetized rotor pole pieces may be fixed in position without any magnetic forces between adjacent poles interfering with the rotor assembly process. This post-magnetization technique is especially preferred when the rotor support frame is formed from plastic material in a high speed injection molding process.
Preferably, the permanent magnets are arranged in the rotor support frame so that adjacent rotor poles on each face comprise permanent magnets having opposite polarities. Furthermore, the permanent magnets on the opposite faces of the rotor may be arranged so that the rotor poles on one face are angularly offset from the rotor poles of the other face. The orientation of the permanent magnets depends upon the arrangement of the coil windings in the stator or stators and the configuration of the electrical power supply. Preferably, the permanent magnets extend between the opposite faces of the rotor in a direction parallel to the rotor axis so that pairs of poles on opposite faces of the rotor are formed by end faces of a single magnet.
Preferably, the stator is formed from a laminated toroid. Most preferably, the toroid comprises a wound length of steel strip. Suitable steel includes silicon and nickel steel strip. The steel strip may be pre-punched before winding so as to form slots for receiving the coil windings or slots may be machined in a separate step after the toroid has been wound. As an alternative, the stator may comprise a number of shaped pole pieces secured to a separate stator end plate and spaced radially around the stator end plate so as to provide the slots for the coil windings. These pole pieces may themselves be of a laminated construction. As a further alternative, the stator may be formed by machining slots in a solid toroid. The entire stator, including the coil windings, may be encased within an epoxy resin or other suitable material so as to provide mechanical rigidity and environmental protection.
Preferably, the permanent magnets in the rotor are formed so that the rotor poles have sides which extend radially outwards from the center of the rotor. Likewise, it is preferred that the slots in the stator are formed so that the sides of the slots extend radially outwards from the center of the stator.
Preferably, the angular width of the slots separating adjacent stator poles is equal to the angular width of the stator poles.
A number of different ratios of the number of rotor poles to the number of stator poles may be used providing the required pole pattern overlap condition is produced. In particular, the number of rotor poles may exceed the number of stator poles or vice versa. For a 3-xcfx86 motor, the number of stator poles must be divisible by 3. Suitable pole ratios for a 3-xcfx86 motor include 6:8 and 6:4 (the ratio of the number of stator poles to the number of rotor poles). Where a large number of rotor poles are required in the design of a motor, the faces of the stator poles may be provided with a number of slots so as to form sub-poles with sides which extend radially outwardly from the center of the stator. The sub-poles are milled so that the angular width of a stator sub-pole is equal to the angular width of one rotor pole. The sub-cut poles provide control over a wide range of operating speeds as the angular displacement of the rotor for each incremental step is reduced. This can be improved further by forming the edges of the stator poles so that the edges overhang the slots between adjacent stator poles. The pole overlap condition is achieved by considering the slots between adjacent stator poles as xe2x80x9cfictivexe2x80x9d poles.
Preferably, the electrical power supply and motor controller are arranged to energize the coil windings in a bipolar mode. Alternatively, the coil windings may be energized in a sequential unipolar fashion.
Cooling of the motor may be assisted, for example, by the provision of a fan.
In one preferred example, the rotor poles each comprise a permanent magnet arranged so that adjacent rotor poles have opposite polarities, wherein coil windings of adjacent stator poles are connected to a different phase and adjacent coil windings connected to the same phase are wound in the opposite sense, and wherein the number of stator poles is a factor of 12 and the number of rotor poles is a factor of 18. The pole overlap pattern provides an extremely efficient motor, generating high torques at low speeds. The motor may be driven by a conventional two phase supply. Alternatively, a single phase supply may be used, with a reactive element used to provide a phase difference between the signals supplied to the two sets of coil windings. Preferably, the permanent magnets extend between opposite faces of the rotor in a direction parallel to the rotor axis so that pairs of poles on opposite faces of the rotor are formed by end faces of a single magnet.
According to a second aspect of the present invention, an axial pole motor comprises a stator and a rotor which face one another across a plane perpendicular to the rotor axis, the stator having coil windings substantially uniformly circumferentially spaced and lying in a plane substantially perpendicular to the rotor axis defining a number of stator poles, the rotor also including a number of rotor poles each comprising a permanent magnet and arranged so that adjacent rotor poles have opposite polarities, wherein coil windings of adjacent stator poles are connected to a different phase and adjacent coil windings connected to the same phase are wound in the opposite sense, and wherein each rotor pole spans only N stator poles, where N is the number of phases.
This motor has such a high detent torque that it provides its own integral fail-safe brake. Preferably, the motor is a 2-xcfx86 motor and therefore each rotor pole spans two stator poles.